A cell in present cellular radiotelephone communications systems typically includes six directional antennas, centrally located in the cell, each radiating into a 60.degree. sector of the cell. A plurality of these cells is combined to form a cellular radiotelephone communications system. This cellular system, covering a metropolitan area, allows mobile traffic to communicate on landline telephone networks while moving through the area.
Communication between the mobile traffic and the celluar system is accomplished using either digital or analog transmission techniques. The digital method digitizes the information before transmission. The analog transmission technique is the prevalent method in use, while digital is now being introduced. Signals transmitted by digital transmission can tolerate a lower threshold of quality, referred to in the art as the Carrier to Interference ratio (C/I), than analog transmitted signals. What is lost in C/I performance can be made up through coding gain.
C/I is a ratio of the signal strength of the received desired carrier to the signal strength of the received interfering carriers. A number of physical factors can affect C/I in cellular systems: building, geography, antenna radiation patterns, mobile traffic transmitting power, and mobile traffic location within the cell.
Due to the low power of the cell's transmitters, the same frequency can be reused in other cells, referred to as co-channel cells, in the same metropolitan area. There are, however, constraints on the location of the co-channel cells. Even though the transmitters are typically low power, placing co-channel cells too close may cause interference. Greater frequency reuse allows more mobile traffic to use the cellular system.
The frequency plan in a symmetrical cellular system, in the sense of which channels should be assigned to each cell, begins with two integers, i and j, that are called shift parameters. The frequency plan is established by starting with a reference cell and moving over i cells along the chain of cells. After reaching the i.sup.th cell, a counter-clockwise turn of 60.degree. is made and another move of j cells is made. The j.sup.th cell can safely be a co-channel cell. The frequency plan can also be established by moving j cells before turning i cells or by turning 60.degree. clockwise instead of counterclockwise. After all the possible co-channel cells of the initial cell are laid out, another reference cell is chosen and the procedure repeated. This entire procedure is repeated as often as necessary to establish the frequency plan of the entire metropolitan cellular system.
The cells thus established by the above procedure form a reuse pattern of i.sup.2 +ij+j.sup.2 cells. The number of cells in this reuse pattern is a predominant concern of the cellular industry since this number determines how many different channel groups can be formed out of the frequency spectrum allocated to cellular radiotelephones. A low number of cells in a reuse pattern means more channel groups can be formed and more users accommodated.
Presently, a four cell reuse pattern is one of the densest frequency reuse patterns that produces an acceptable C/I for analog systems (U.S. Pat. No. 4,128,740 to Graziano, assigned to Motorola, describes such a four cell reuse pattern).
Graziano teaches that frequency reuse is a function of antenna beam width antennas and the antennas' spatial relationship with one another. In order to increase frequency reuse, the antenna beam is narrowed from 120.degree. to 60.degree.. Since a 120.degree. antenna beam covers a wider area, it will interfere with more co-channel cells than a 60.degree. antenna beam. A narrower antenna beam, as illustrated in FIG. 1, reduces the area covered by the antenna's radiation pattern. By reducing the beam width and spatially arranging antennas, while remaining cognizant of the power directivity of each and the cumulative power of co-channel interferers, allows greater frequency reuse.
The frequency reuse pattern described in Graziano is a symmetrical reuse pattern. The symmetrical pattern is obtained using the cellular layout procedure described above. In this configuration, each co-channel cell is substantially equidistant from the other co-channel cells. With a symmetrical configuration, cell layout is limited by the frequency reuse equation in the number of different reuse configurations possible.
There exists a need, therefore, to decrease the number of cells in a cellular reuse pattern thereby increasing the number of times a frequency can be reused.